This Project is focused on understanding the molecular mechanisms involved in vascular calcification and osteoporosis. Matrix GLA protein (MGP) was strongly implicated in the pathogenesis of vascular calcification when an MGP knockout mouse was found to have extensive vascular calcification. Based on this mouse model, MGP might be thought to be an inhibitory factor in vascular calcification. However, in calcified lesions of mice that have a normal MGP gene and in human lesions, MGP expression is postively correlated with the degree of lesion calcification. During the current grant period we demonstrated that MGP regulates bone morphogenetic protein (BMP-2), During the next grant period we will determine the molecular basis for the regulatory role of MGP in the artery wall and its interaction with BMP-2. Other work from this Project during the current grant period provided important clues as to why many patients with progressive vascular calcification also have progressive osteoporosis. Oxidized lipids were shown to promote calcification of calcifying vascular cells (CVC) but inhibited the osteoblastic differentiation and mineralization of marrow stromal cells that are the precursors to mature bone osteoblasts. This was true whether the oxidized lipids were added in vitro or were produced by feeding atherosclerosis suceptible C57BL/6J (BL6) mice an atherogenic diet. These oxidized lipids were also shown to promote osteoclastogenesis and osteoclast activation in vitro. In vivo, feeding an atherogenic diet to atherosclerosis susceptible BL6 mice produced a dramatic reduction in bone mineral density and bone mineral content. Feeding an atherogenic diet to atherosclerosis resistant C3H/HeJ mice causes the same degree of hyperlipidemia as in BL6 mice, but there was no significant reduction in either bone mineral density or bone mineral content in the C3H/HeJ mice. In the next grant period we propose to determine the molecular mechanisms for these observations and we will determine if high density lipoproteins (HDL), components of HDL, and mimetics of HDL will protect against bone loss in mouse models of atherosclerosis. These studies may identify potential new therapeutic targets in vascular calcification and osteoporosis.